SE-A-468132 reveals an exhaust valve mechanism of the above type which, together with a special type of cam shaft with an extra small cam lobe, can be used to increase the braking power of the engine. The extra cam lobe is dimensioned so that its lift approximately corresponds to the normal valve play in the valve mechanism. By reducing, by means of the valve play absorbing mechanism, the valve play to zero, an extra lift can be achieved of the exhaust valve corresponding to the normal valve play, during a suitable time interval. For example, the extra cam lobe can be placed relative to the ordinary cam lobe, so that an extra exhaust valve lift is obtained during a latter portion of the compression stroke, resulting in loss, during the compression stroke, of a portion of the compression work, which cannot be recovered during the expansion stroke. The result is an increase in the engine braking power.
An engine with such an arrangement for increasing-the braking power has, however, a somewhat lower efficiency than a corresponding conventional engine. This is due to the fact that the maximum lift of the exhaust valves at zero valve play is utilized for brake mode and not for engine mode, which means that engine mode must be compensated by lower lift when the valve play absorbing device is not activated, which also means that the exhaust valves close earlier than if a maximum lift can be used for engine mode. An additional disadvantage of the known arrangement is that it requires a special cam shaft.